Work package 3: experimental rock physics (NOC)

Summary

The research in work package 1 and work package 2 will be underpinned by new experimental work on samples representative of the Sleipner, Snøhvit and In Salah CO₂ reservoirs. We have developed new capabilities for providing accurate elastic wave properties of 5 cm diameter (2 – 3 cm long) rock samples. These include elastic wave (ultrasonic) velocity and attenuation anisotropy measurements at high effective stress (< 60 MPa); analysis of synthetic fractured porous rocks with controlled fracture geometry and degree of silica cementation; X-ray CT imaging of rock samples, and handling of a range of rock types from tightly-cemented sandstones to unconsolidated sands with a range of pore fluids including mixed phases. We will measure elastic wave velocity and attenuation anisotropy as a function of stress (including differential stress), temperature (up to 50°C) and pore fluid (e.g. brine/CO₂ mixtures in gaseous, liquid and critical state), as well as fracture state (open/closed), fracture density and orientation.

To achieve this, we will build a new experimental rig, based on our recent innovations, covering a wide range of reservoir types from the very challenging issue of obtaining measurements in unconsolidated sands (such as at Sleipner), to much harder fracture-prone rocks (such as at In Salah). Supporting task 2.2, we will also carry out experiments on synthetic rock with controlled crack geometry, using the method developed at NOC (Tillotson et al., 2012), involving different saturations of CO₂ and different pressure regimes, aiming to construct the full elastic tensor. Work will be divided into five tasks:

• task 3.1: CO₂ rig development
• task 3.2: manufacture of synthetic fractured silica-cemented sandstones
• task 3.3: ultrasonic anisotropy measurements on the synthetic fractured rocks as a function of CO₂/brine saturation
• task 3.4: ultrasonic anisotropy measurements on CO₂/brine saturated sand samples
• task 3.5: integration of the results with other work packages

This is an ambitious laboratory programme that seeks to provide the necessary rock physics data for improved field seismic analysis and modelling. We have already made significant progress in key methods that will facilitate this work, such as developing a technique to manufacture synthetic cracked rocks (17), ultrasonic experiments on very weakly cemented sands (Best, Sothcott and Hicks, 2008; Best and Sothcott, 2010) and handling of multiphase fluids in sands and sandstones (Best, Priest and Clayton, 2010; Sothcott et al., 2007) as well as novel electrical methods for imaging sample heterogeneities (North et al., in press).